Scientists analyzing brown bullhead catfish in Vermont’s Lake Memphremagog have discovered a rare skin melanoma that passes directly between animals like a parasite, marking the fourth known transmissible cancer in the animal kingdom and the first ever identified in a freshwater fish species.
For more than a decade, anglers pulling brown bullhead out of Lake Memphremagog noticed a startling blemish. The fish frequently hauled up with thick, inky-black patches and raised growths smeared across their bodies, whiskers, and mouths. What anglers and biologists first began documenting in 2012 quickly saturated the cross-border water shared by Vermont and Quebec. By 2014, roughly one in three fish displayed the dark lesions, prompting a multi-year scientific hunt for the culprit.
Genetic Sequencing Uncovers a Clonal Infectious Agent
Researchers initially suspected that environmental contamination was driving the growths. Because brown bullhead live directly on the lakebed and feed along the sediment, investigators tested the watershed for industrial pollutants, heavy metals, and runoff stirred up after Tropical Storm Irene tore through in 2011. Biologists also searched extensively for a viral pathogen or retrovirus, similar to the viruses that cause skin sarcomas in walleye.
Those hypotheses fell flat. Whole-genome sequencing performed by researchers at the University of Vermont revealed an entirely unexpected biological mechanism. When geneticists compared DNA from the tumors against healthy tissue from the host fish, the tumor cells showed a startling pattern: they were far more closely related to each other than to the fish carrying them, sharing hundreds of thousands of genetic variants completely absent from their hosts.
“It’s a clonal cancer. It’s not driven by a bacteria or a virus. It’s the infectious agent itself, and it’s moving from fish to fish.”
Julie Dragon, associate professor of microbiology and molecular genetics at the University of Vermont
This genomic signature identifies a clonally transmissible cancer, where living cancer cells detach from one host, survive in the environment or through direct contact, and establish themselves in a new animal. Until this finding, published in the journal Nature, scientists had confirmed naturally occurring transmissible cancers in only three other groups of animals: dogs, Tasmanian devils, and marine bivalves such as clams and mussels.
Spawning Behavior and Ecological Questions
While the genetic sequencing confirmed how the cancer behaves, researchers are still investigating how the cells travel between individual fish. Because brown bullhead are bottom-dwelling fish that interact closely with sediment, scientists point to seasonal spawning habits as a likely vector for transmission.
During the spawning season, bullhead aggregate in dense clusters in shallow waters, creating prime conditions for physical contact and the transfer of living cells.
The geographic distribution of the disease presents a distinct puzzle. Although the binational lake spans more than 30 miles and the cancer has saturated Lake Memphremagog, surveys across other regional watersheds show a stark contrast.
Public Health Reassurances and Regional Spread
State and federal agencies emphasize that the disease poses no known risk to humans.
Subsequent genetic sampling across broader waterways indicates that Lake Memphremagog is not entirely isolated.
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